Rechargeable batteries (some times referred to as secondary batteries) used for powering consumer electronic mobile devices are the subject of much research and development. Mobile devices such as smart phones, tablet computers, and laptop or notebook computers are being developed with increasing functionality and performance, which has led to a demand for lower cost yet more powerful rechargeable batteries. There is also a need that the rechargeable battery used in advanced mobile devices be as small as practical, read as thin as possible.
Currently, lithium secondary batteries having lithium ion and lithium ion polymer chemistries have the needed energy density and the requisite thin form factor to be the preferred choice for powering a mobile device. Recently, much interest has been shown in a lithium battery cell whose electrochemical structure (which includes a pair of electrodes, separator, and electrolyte) are encased in a flexible, lightweight pouch. The cell pouch is a flexible, sealed casing in which a single cell is contained. The pouch is flexible in that it may be expected to swell during charge or discharge or as the cell heats up. It is sealed in that it prevents any components of the electrochemical structure of the cell, including the electrolyte in particular, from leaking out.
Typically, the pouch wall has a foil construction that includes a metal foil, which is insulated from the cell's electrochemical structure and from the outside world by being laminated between at least two insulating layers. The metal foil laminate acts as a protective moisture barrier, preventing water and moisture from entering the pouch. The metal foil is thus external to and electrically insulated from the electrochemical structure of the cell that is contained in the pouch. The metal foil is also electrically insulated on the outside of the cell.
The metal foil of the pouch should not be electrically biased. For instance, the metal foil should at all times remain electrically “floating” or sufficiently disconnected (electrically) from any electrical power source. This desired characteristic may be verified during production or laboratory testing of the cell, by performing a dc impedance measurement using an ohmmeter, during which an ohmmeter probe is temporarily placed into contact with the metal foil. This is to ensure that the foil is electrically insulated; if the measurement reveals sufficiently low impedance between the foil and a power terminal of the cell, then the cell is identified as being defective.